Mats to be tested consist of fibers and/or wood chips and are actually preferably formed into chip boards, fiber boards, oriented strand boards (“OSBs”) and similar boards in continuous double band presses. Today's current, continuous double band presses are isochoric, i.e., they operate by maintaining a predetermined distance between the press plates. The double band presses are provided with steel belts that run in opposite directions relative to one another and that compress the mat to achieve a final thickness. Foreign matter and zones in the mat that cannot be compressed to this final thickness may cause bulges, cracks or even cuts to be formed in the steel belts and may even damage the roller bar or roller chain systems supporting the steel bands and the heating plates.
To avoid such damage, it has become known in board manufacturing to make use of metal detectors which sense magnetizable and non-magnetizable metal pieces in the mat. Upon detection of such metal pieces, the forming belt, which is divided transverse to the direction of motion, is parted in the direction of motion and the defective portion of mat is evacuated into a discharge chute. After closure of the forming belt, the manufacturing process is carried on with a flaw free mat. Magnetizable metal pieces are also removed from the mat by magnets.
CA 1 202 431 A teaches to dispose, on one side of a plate-shaped product, one radiation source the output beam of which is fan-shaped. The width of the beam extends transverse to the direction of motion of the product. On the other side of the product, detectors are arranged in only one row on an arc of a circle, the center of which is the radiation source. The detectors are mounted in alignment with the fan-shaped beam. A distant positioned detector receives radiation from a radiation source that has not penetrated the product. This distant positioned detector serves to automatically calibrate the known device. This device is intended for determining the weight per unit area of the product. If the density of the product is constant, the thickness may be determined from the weight per unit area. On the other hand, if the thickness of the product is constant, the density of the product may be deduced from the weight per unit area.